Coil assembly for testing of rods or tubes



May 31, 1960 F. M. o. FOERSTER 2,939,071

COIL ASSEMBLY FOR TESTING OF' RODS OR TUBES FiledrAug. 7, 195'? l l, lv,

/33 50 l f h A' C' PHASE AMPLIFIER -r GENERATOR/W SHIFTER J- (54 f5/ LI9 I46 [nz/E17 fr F/EoR/CH M. 0. /LBERS TER www 4W! MM2/5.

COIL ASSEMBLY FOR TESTING OF RODS OR TUBES Friedrich M. O. Foerster, Grathwohlstr. 4, Reutlinger, 'Germany Filed Aug. 7, 1957, Ser. No. 676,760

4 Claims. (Cl. 324-34) This invention relates to a coil assembly for testing of rods or tubes and more particularly to an assembly in which an elongated cylindrical object, either a rod or a tube, is placed in the magnetic eld of the assembly, with cracks or other changes in physical properties being detected by the effect on the eld of the coil assembly.

It has heretofore been proposed to detect cracks in' rods or tubes by means of an exciting coil in surrounding relation to the rod or tube, to which an alternating current is applied, with a pair of receiver coils located underneath the exciting coil. The receiver coils are located in axially spaced relation and the outputs thereof are compared, so as to compare the magnetic characteristics of one portion of the rod r tube with a portion of the rod or tube spaced axially therefrom.

It has been found that with such an arrangement, there is a movingeffect such that when the rod passes through the irst receiver coil, the rod is coming from a eld-free space while when it passes through the second coil, the rod is coming from a region having a strong field. This results in a difference in the indication and,

in addition, the difference varies with the speed of movement of the rod. In order to eliminate this movingeffect, it has been proposed to use two coil pairs which are wound one upon the other and are connected in opposition. These coil pairs are very ditlicult to adjust and in addition reduce the sensitivity considerably.

The above-described type of system, and other prior systems vof a similar nature, have suffered from a lfurther disadvantage in that they have a very low penetration depth. That is, they Will detect cracks which are close to the surface of the rod or tube, but cannot readily detect cracks which are located toward the center of the rod or tube.

It is therefore a primary object of this invention to provide an improved coil assembly for the testing of rods or tubes, in which the above described moving-etfect is minimized.

A further object of this invention is to provide a coil assembly having improved penetration characteristics.

According to this invention, a coil assembly is provided which comprises two sepa-rate transformers each having primary and secondary coils, with the transformers being spaced a substantial distance apart so that the etect of one upon the operation of the other is minimized. A highly important feature of the invention is in the provision of at least one ring of conductive material between the two transformers. As the rod or tube moves from one transformer through the ring, currents are induced in the ring by any magnetization of the rod or tube, which opposes such magnetization and electively reduces the magnetization to zero. Accordingly, as the rod or tube enters the second transformer, it is substantially completely demagneh'zed, and the previously described moving-elfec is eliminated. A ring of conductive material is preferably located in advance of each transformer, so as to insure that the rod or tube will be demagnetized as it enters each transformer. The coil "ice assembly is preferably such that it can be operated with movement of the rod or tube in either direction, and a conductive ring is hence preferably located on each end of each transformer.

According to a further feature of the invention, the transformers are of very short axial length, as compared to the diameter of the rod or tube under test. The axial length :of the transformer is on the order of 2.5 times the diameter of the rod or tube and is preferably not greater than ve times the diameter of the rod or tube.y

It has been found that with this relationship, the penetration depth is greatly increased. This advantageous eect is enhanced by the use of the conductive rings which are preferably located closely adjacent the ends of the transformers, to reduce the effective length.

The increased penetration depth, which resultswith a short axial length, is apparently due to the lfact that the apparent permeability decreases as the length decreases, and from the decreased apparent permeability, it should be obvious that the penetration depth will increase. This arrangement has a further advantage in that it is less sensitive to permeability variations which are caused, by cold Working.

This invention contemplates other and more specilic objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate a preferred embodiment and in which:

Figure 1 is a cross-sectional View through a coil assembly constructed according to the principlesof this invention; and

Figure 2 is a schematic circuit diagram illustrating a testing system utilizing the coil assembly of Figure l.

Reference numeral 10 generally designates a coil assembly constructed according to the principles of this inven- Y tion. The assembly 10 comprises a pair of transformers 11 and 12 of substantially identical construction. VThe transformer 11 comprises a coil form 13 having a pair of annular grooves 14 and 14a in which are wound portions 15 and 16 of a primary coil. The form 13 further has a groove 17 intermediate the grooves 14 and 14a and of smaller diameter in which is wound a secondary coil 18. The transformer 12 similarly comprises a coil form 19` having grooves 20 and 21 receiving portions 22 and 2,3 of a primary coil and a groove 24 which receives a coil 25. 'Ihe coil forms 13 and 19 are mounted within a tube 26. Also mounted within the tube 26 are a lirst pair of rings 27` and 28 at the opposite ends of the form 13 and a second pair of rings 29 and 30 at the opposite ends of the form 19. Disposed between the rings 28 and 29 is a spacer tube 31. The rings 27-30 are preferably of a highly conductive material, most preferably steel although copper and other materials may be used.

It will be apparent from the drawing that the forms 13 and 20, the spacer tube 31 and the rings 27-30 are coaxially mounted and together they provide a generally cylindrical opening indicated by reference numeral 32, through which the rod or tube under test is passed.

Referring now to Figure 2, the portions 15 and 16 of the primary coil of the transformer 11 are connected in series, and likewise the portions 22 and 23v of the primary coil of the transformer 12 are connected in series, with the series-connected primary coil portions being connected in parallel and to the output terminals 33 and 34 of an A.C. generator 3S. The secondary coils 18 and 25 are connected in series and to a primary coil 36 of a transformer 37 which has a secondary coil 38 connected to the input terminals 39 yand 40 of 'an Iampliiier 41. The amplifier 41 has output terminals 42 and 43 connected to one pair of deflection plates of 'a cathode raytube 44 as diagrammatically illustrated. The other pair of deliection plates of the cathode ray tube 44 are portance in obtaining a symmetrical construction.

connected to output terminals 45 and 46 of an amplifier 47- lraving input terminals 48 .and 49 connected to output terminals of an adjustable phase shifter 52 which has input terminals 53 and 54 connected to the output terminals 33- and 34: of the A.C. genera-tor 35'.

InZ operation, the instrument operates' to compare the portion of they rod or tube under test whichlies. within the transformer 11V to the portion of the rod. or. tube which liesf Within the transformerl 12. If the two portionsof the rod. or tube have identical; characteristics, there will be no 'out-put from the secondary coils 18 and 25, the twobeing connected in phase opposition, and a line Will appear on. the face of the cathode: ray tube 44;V If, however, there is a `difference in characteristics' between the twoportions `of. they rod or. tube yunder test which lie Within the transformers 11 and. 12, anr output will be developed in the secondaryv coil circuit which was amplified; `and applied. to the cathode ray tube to produce a Lissajousl figure, yfrom which itis; possible to analyze the difference in character ofV the. two portions of the rodor tube under test. To expedite this analysis; the phase shifter S2 may be adjusted.

lt will be apparentthat if, for example, the tube. or rod under test is. moving. from. left to right as viewed in Figure 1, if any' portion of the rod or tube is initiallyv magnetized, sucl'r. magentization will. produce strong; currents in the ring 27 which Will produce a magnetic flux inopposition. to the flux produced. by the magnetized. portionof: the rod or tube, yand the rod or tube Willv be substantialy demagnetized. Hence, each portion of' the rod or tube wil-1 be demag-netized as it enters the transformer 1'1. AsV each portion of the rod or tube leaves the transformer 11, it; will be demagneti'zed not only by the ring 28. but-also by the ring 29 to insure. that asit enters the second transformer 12, it will Iagain be substantially completely demagnetized. Each portion of. the rod or tube'will again be demagnetized by the ring 30.` This,V

in itself, lis not of. importance but it is desirable. thatithe assembly' be usable with. movement of the rod orftube in either direction. In addition, the rings are within the fields of thel transformers, and. it is highly desirable, .if not essential, that the transformers be of substantially the. same construction.

It will be noted that the transformers have a very Vshort axial length, on the order of 2.5 times the ydiameter of therod or tube under test. Preferably, the axial length should not exceed five times they `diameter of therod or tube under test. The effective axial length is, ofl course;

further reduced by the use of the rings 27'-30. The y advantages of the short axial length are pointed. out above. It may be noted. that this has afurther advantage in that itis possible to test closer to the vends of :the rods or. tubes.

The division of the primary windings into two por.- tions With the secondary coils therebetween is of im.- It is of further advantage to have the secondary coilsfof smaller' diameter than the primary coils, 'as illustrated, to obtainV better penetration.

It may be noted that the system may be operated at 4any desired frequency; Itis possible to use 25, 50` or 60 cycle alternating current, in which case the primary coils and the phase shifter may be` connected toy a power line. However, when .itis desired to test rods or tubes moving at a fastrate,4 it is desirable to increase the frequency of operation. In such a case, the A.C. generator may :be yin the form of a vacuum tube oscillator.

It should further be notedY that the system is not limited to. the testing. of rods or tubes of magnetic material and may be used to test any conductive materials. It may, for example, be used to test copper rods or tubes.

It will be understood that modifications land variations may be effected without departing from the spirit and scope of the novel concept of this invention.

. I claim as my invention:

1. In laplaar-.atus for testi-ng elongated cylindrical objects, generally tubular ,coi-lr formv means arranged to receive the object to be tested, a first transformer comprising primary land secondary coils wound'l on said coil form means, `a second transformer comprising primary and secondary coils Wound on said coil form means in axially spaced relation to said primary and secondary coils of said rst transformer, means for applying an alternating current to said primary coils, at least one ring of conductive material between said transformers.

2. In. apparatus for testing elongated cylindrical objects, generally tubular coil form means arranged to receive the object to be tested, a first. transformer comprising primary and secondary coils wound on said coil form means, aV second transformer comprising primary and secondary coils Wound on said coil form means in axially' spaced relation tor said. primary' and secondary coils of said rst transformer, means for' applying. an alternating current to said primary coils, a pairY of rings f conductivematerial located in `axially spaced relation between. said transformers and 'in' closely spaced relation to the adjacent ends of said transformers.

3'. In apparatus for testing .elongated cylindrical objects, generally tubular coil form means arranged' to re-Y ceive the object to be testedfa. first` transformer comprising. primary and. secondary coils Wound on said coil form means, a second. transformer comprising primary and secondary coils Wound. on said coil` form means in axially spaced relation to .said primary and .secondary coilsof said first' transformer, means for applying an alternating current to said. primary coilsr pair of rings of conductive material located lin axially spaced rel-ation' between' said transformers and `in closely spaced relation to the adjacent ends of said transformers, and a second pair of rings respectively located in closely spaced relation to the other ends of said transformers.

4. In. apparatus for testing. elongated cylindrical objects, generally'tubular coil form means arranged to receive the object to be. tested, a first. transformer comprising primary and secondary'coilszwound on said coil form means, a second transformer comprising. primary and secondary coils woundon said coil form means in axially spaced relation to saidprimary vandsecondary coils of said lfirst transformer, means for applying an alternating current to said. primary coils, and a. plurality of ringsv of conductive material disposed incoaxial relation to said coil foi-m means with rings being located in closely spaced relation to both ends of each transformer to limit the eifective axial' lengths of the-transformers.

References Cited in the tile of thispa'tent' Irwin Mar. '30,1 1954 L Lili 

